Murray & Roberts Cementation has taken an innovative approach to a project that will see the establishment of two ventilation shafts – one upcast and one downcast – at Bokoni Platinum mine on the north-eastern Limb of the Bushveld Complex in Limpopo. For this project, the mining infrastructure, construction and services company has departed from the conventional blind sinking option in favour of bulk earthmoving and excavation methods.
Project manager Dirk Visser explains that Murray & Roberts Cementation conducted a trade-off study during the tender phase which revealed that taking a bulk earthmoving and excavation route would be more cost effective. It is also the safer of the two options and will reduce the manpower requirement substantially, with personnel on site at project peak likely to be less than 45 people.
Theupcast 88metre 2East0 ventilation shaft will havea completed diameter of 4.6metres and the 350 metre downcast 6West1 ventilation shafta completed diameter no less than 5.9metres. Both shafts are being constructed using the same method, with variations on the dimensions and quantities and the project is scheduled for completion in August 2014.
On both shafts Murray & Roberts Cementationbegan by excavating and removing the soil overburden from depths of about 24 to 50 metres to reach competent rock. Portions of the initial excavation process were carried out simultaneously and, after the overburden was excavated, contiguous piles were sunk from the bottom of the open excavation to the refusal depth.
After piling, a curtain grout pre-cementation method will be applied to a depth of 80m. A concrete reinforced pile cap will be constructed as a foundation for the raisebore machines. Once raiseboring has been completed precast concrete rings, with diameters that correspond with the diameters of the shafts, will be installed from the bottom of the installation back to natural ground level. Concurrent with the precast concrete ring installation, the previously excavated soil materials will be backfilled and compacted to natural ground level.
A remote shaft lining machine will be used to inspect the shaft and, based on the client’s recommendation, a 100mm fibre lined Drycrete shotcrete will be applied to the shaft walls in layers of 25mm.
The Murray & Roberts Cementation team has been mindful of the wellbeing of the local community, especially when it comes to using community roads where the large earthmoving machinery has to interface with the community’s vehicles. Overburden, being used for backfill purposes during the contract, is being stockpiled well away from the local community’s dwellings.
Safety and quality
"Safety is fundamental to the success of any project and Murray & Roberts Cementation is committed to ensuring that all works are carried out in accordance with statutory requirements, site-specific instructions and internal group standards," Visser says. "The site is being operated under the Business Culture Triangle approach and all personnel are being included in an induction programme, together with ongoing training and safety awareness campaigns.
"In terms of quality, we give our complete commitment to our Quality Assurance Programme and to developing and maintaining ISO 9001 requirements, with the objective of promoting client confidence, eliminating waste and ensuring an ongoing successful business unit," Visser concludes.